my first build

[irving2008]

they do hardened high carbon round rail
http://www.worldofcnc.com/products.asp?recnumber=14
from reading other threads i think i need 16mm for the bed. they have this...w16 (h6) at £1.90 per 100mm. now i assume that product is 16mm diameter, not sure what the h6 means though.

The h6 is the tolerance on the diameter I believe which is as you surmise 16mm... However these are not supported rail. They will deflect and bounce on long runs and are not really suitable for more than 300mm on a lightweight machine. Although they can be used where accuracy is not paramount, really you need to be looking at fully supported rail. If you want to use unsupported rail you need to be considering 20mm or bigger...

also have these http://www.worldofcnc.com/products.asp?recnumber=17
at £5.40 for the LME16UU i assume that is to suit a 16mm rail.

That is correct, however these round units are hard to use without the matching pillow block

am i right in thinking that i can get away with thinner rails in the gantry?

Yes, but 16mm unsupported rail will deflect 0.2 - 0.3mm with a 6kg router/z-axis in the middle on a 600mm run, so this limits your depth accuracy and surface finish.

i am now considering putting off the purchase of the controller/steppers untill maybe february so i can spend some money on these. does anyone know if these guys accept paypal.

MD accept PayPal, but dont expect fast turnaround... you could be looking at a few weeks based on some recent experience by members... they are a tad overloaded...